1. Field of the Invention
The present invention relates to a display panel in which a light emitting element is formed on a substrate and is sandwiched between the substrate and a cover member. The invention also relates to a display module obtained by mounting IC to the display panel. In this specification, the display panel and the display module are called by a generic term, light emitting device. The present invention also relates to a method of driving the light emitting device and to electronic equipment using the light emitting device.
2. Description of the Related Art
Being self-luminous, light emitting elements eliminate the need for back light necessary in liquid crystal display devices (LCDs) and therefore can make thinner devices. In addition, light emitting elements have higher visibility and no limitation in terms of viewing angle, and these are the reasons for attention that light emitting devices using light emitting elements are receiving in recent years as display devices to replace CRTs and LCDs.
A light emitting element has a layer containing an organic compound that provides luminescence (electro luminescence) generated upon application of electric field (hereinafter referred to as organic compound layer), as well as an anode layer and a cathode layer. Luminescence provided by organic compounds is divided into light emission upon return to base state from singlet excitation (fluorescence) and light emission upon return to base state from triplet excitation (phosphorescence). Both types of light emission can be used in a light emitting, device of the present invention.
All the layers that are provided between an anode and a cathode are an organic compound layer in this specification. Specifically, the organic compound layer includes a light emitting layer, a hole injection layer, an electron injection layer, a hole transporting layer, an electron transporting layer, etc. A basic structure of a light emitting element is a laminate of an anode, a light emitting layer, and a cathode layered in this order. The basic structure can be modified into a laminate of an anode, a hole injection layer, a light emitting layer, and a cathode layered in this order, or a laminate of an anode, a hole injection layer, a light emitting layer, an electron transporting layer, and a cathode layered in this order.
In this specification, making a light emitting element emit light is expressed as driving the light emitting element. The light emitting element as defined herein is an element that is composed of an anode, an organic compound layer, and a cathode.
Methods of driving a light emitting device having a light emitting element are roughly divided into analog driving methods and digital driving methods. Digital driving is deemed more promising in view of transition from analog broadcasting to digital broadcasting since it enables the light emitting device to display an image using a digital video signal that carries image information as it is without converting the signal into an analog signal.
Among the driving methods that obtain gradation display by binary voltages of digital video signals, there is a time division driving method in which lengths of time for lighting a pixel are controlled to obtain gradation display.
In the time division driving method, one frame period is divided into a plurality of sub-frame periods. In each sub-frame period, to be lit or not is determined for the respective pixels in accordance with digital video signals. The accumulated lengths of sub-frame periods during which a pixel is lit with respect to the length of the entire sub-frame periods in one frame period determine the gradation of that pixel.
Described below are the structure of a pixel portion in a common light emitting device, and a driving method thereof.
FIG. 17A is an enlarged view of a pixel portion 7000 in a common light emitting device. The pixel portion 7000 has source signal lines S1 to Sx, power supply lines V1 to Vx, and gate signal lines G1 to Gy.
A region provided with one of the source signal lines S1 to Sx, one of the power supply lines V1 to Vx, and one of the gate signal lines G1 to Gy corresponds to a pixel 7001. The pixel portion 7000 has a plurality of pixels that are arranged to form a matrix.
FIG. 17B shows an enlarged view of the pixel 7001. The pixel 7001 has a source signal line Si (one of the source signal lines S1 to Sx), a power supply line Vi (one of the power supply lines V1 to Vx), and a gate signal line Gj (one of the gate signal lines G1 to Gy).
The pixel 7001 also has a switching TFT 7002, a driving TFT 7003, a light emitting element 7004, and a capacitor 7005.
The switching TFT 7002 has a gate electrode connected to the gate signal line Gj. The switching TFT 7002 also has a source region and a drain region one of which is connected to the source signal line Si and the other of which is connected to a gate electrode of the driving TFT 7003 and to the capacitor 7005.
The driving TFT 7003 has a source region and a drain region one of which is connected to the power supply line Vi and the other of which is connected to a pixel electrode of the light emitting element 7004. The power supply line Vi is connected to the capacitor 7005.
The light emitting element 7004 is composed of an anode, a cathode, and an organic compound layer placed between the anode and the cathode. If the anode is in contact with the source region or the drain region of the driving TFT 7003, the anode serves as the pixel electrode whereas the cathode serves as an opposite electrode. On the other hand, the cathode serves as the pixel electrode whereas the anode serves as the opposite electrode if the cathode is in contact with the source region or the drain region of the driving TFT 7003.
The opposite electrode of the light emitting element 7004 is provided with a given electric potential (opposite electric potential). The power supply line Vi is provided with a given electric potential (power supply electric potential). The power supply electric potential and the opposite electric potential are supplied from a power source provided in an external IC or the like to the display panel.
The light emitting device structured as shown in FIGS. 17A and 17B is driven by the time division driving method to display an image. The operation thereof is described next with reference to FIG. 18. In the time division driving method, one frame period has a plurality of sub-frame periods. FIG. 18 shows points at which sub-frame periods are started in the light emitting device structured as shown in FIGS. 17A and 17B, and the axis of abscissa indicates the time scale whereas the axis of ordinate indicates positions of gate signal lines.
In FIG. 18, one frame period has n (n is a natural number) sub-frame periods SF1 to SFn. In each of the n sub-frame periods, digital video signals equivalent to one bit are inputted to each of the pixels. The digital video signals determine whether the light emitting elements in each of the pixels emit light or not.
To detail the above operation, the gate signal lines G1 to Gy are selected one by one to turn the switching TFT 7002 connected to the selected gate signal line ON. In this specification, a signal line being selected means turning every TFT whose gate electrode is connected to the selected signal line ON.
While each of the gate signal lines is selected, digital video signals equivalent to one bit are inputted to the gate electrode of the driving TFT 7003 from the source signal lines S1 to Sy through the switching TFT 7002 that is ON.
Switching of the driving TFT 7003 is controlled by the digital video signals. When the driving TFT 7003 is turned ON, the power supply electric potential is given to the pixel electrode of the light emitting element 7004, and the difference in electric potential between the power supply electric potential and the opposite electric potential causes the light emitting element 7004 to emit light. On the other hand, when the driving TFT 7003 is turned OFF, the power supply electric potential is not given to the pixel electrode of the light emitting element 7004 and therefore the light emitting element 7004 does not emit light. Note, in this specification, the state that the light emitting element emits a light is called as “light emitting state”, while the state that the light emitting element does not emit a light is called as “non-light emitting state”.
When the digital video signals are inputted to all of the pixels, one sub-frame period is ended to start the next sub-frame period. The operation described above is repeated and whether or not the light emitting, element 7004 emits light in each pixel is determined for each of the sub-frame periods SF1 to SFn. As a result, levels of gradations of the pixels are controlled and one image is displayed upon completion of one frame period.
The driving method described above needs to put at least n sub-frame periods in one frame period if an image is displayed using n bit digital video signals. Therefore, when the bit number of digital video signals is increased in order to raise the gradation number of an image, sub-frame periods in one frame period is increased in number.
In an ordinary light emitting device, it is preferable to set 60 or more frame periods in one second. If the number of images displayed in one second is less than 60, flickering of an image may be noticeable to a viewer. In order to contain flickering of an image and display a high gradation image without lowering the frame frequency, lengths of sub-frame periods have to be shortened.
However, when lengths of sub-frame periods are shortened, the speed of inputting digital video signals to pixels may become insufficient for the shortened sub-frame periods. Details of this problem are described below with reference to FIGS. 19A and 19B.
FIGS. 19A and 19B show points at which sub-frame periods SF(k−1), SFk, and SF(k+1) (k is an arbitrary natural number) are started in a common time division driving method, and the axis of abscissa indicates the time scale whereas the axis of ordinate indicates positions of gate signal lines. Reference symbol t1 denotes the length of time for inputting digital video signals equivalent of one bit to every pixel in the sub-frame period SFk and t2 denotes the length of the sub-frame period SFk in pixels in each line. One line of pixels have the same gate signal line.
FIG. 19A shows a case of t1≦t2 whereas FIG. 19B shows a case of t1>t2.
In the case of t1≦t2 shown in FIG. 19A, digital video signals equivalent to one bit are inputted to every pixel after the k-th sub-frame period SFk is ended and this input operation is finished before the next sub-frame period, the (k+1)-th sub-frame period SF(k+1) is started. Accordingly, input of digital video signals equivalent of one bit to pixels and input of the next set of one hit digital video signals do not take place concurrently in the same pixel portion.
On the other hand, in the case of t1>t2 shown in FIG. 19B, input of digital video signals equivalent to one bit to pixels is not finished even after the k-th sub-frame period SFk is ended. In other words, input of the next set of one bit digital video signals to pixels has to be started during the first set of one bit digital video signals are inputted to pixels.
When the sub-frame period t2 is shortened until t1>t2 is satisfied in order to raise the gradation number, the device has to be driven as shown in FIG. 19B. However, the light emitting device structured as FIGS. 17A and 17B cannot be driven in the way shown in FIG. 19B. In order to satisfy t1≦t2, shortening the sub-frame period t2 alone is not sufficient and the period t1 for inputting digital video signals equivalent to one bit to every pixel has to be shortened.
To shorten t1, the drive frequency of a source signal line driving circuit that controls input of digital video signals to the source signal lines needs to be high. However, if the drive frequency of the source signal line driving circuit is too high, transistors of the source signal line driving circuit cannot deal with the drive frequency to fail to operate or lose the reliability.